1. Field of the Invention
The present invention relates in general to evaporative emission control systems for internal combustion engines, and more particularly to an evaporative emission control system of the type which is capable of making a leak diagnosis, i.e., capable of detecting if a leak is present in the evaporative control system.
2. Description of the Related Art
An internal combustion engine for current automotive vehicles includes an evaporative emission control system. The evaporative emission control system includes a canister containing activated charcoal to collect and store volatile fuel vapors from a fuel tank during the time the engine is not inning. The evaporative emission control system also includes a purge line or conduit connecting between an intake pipe portion downstream of a throttle valve and the canister. The purge line opens under a predetermined condition after start of the engine to draw fresh air into the canister and purge the canister. The collected volatile fuel vapors are thus drawn from the canister into the intake pipe, for combustion within a combustion chamber of the engine.
In such an evaporative emission control system, if a conduit between the fuel tank and the intake pipe has a leak or the conduit has a connecting portion of which seal is defective, the fuel vapors are released to the atmosphere. To prevent such evaporative fuel emission, there has been proposed an evaporative emission control system which is adapted to make a leak diagnosis, i.e., which is capable of determining if a leak is present in the evaporative emission control system.
A leak diagnosis can be made by closing the above described conduit in a way as to prevent communication between the inside and outside of the conduit, putting the inside of the thus closed conduit into a condition wherein there is a difference in pressure between the inside and outside of the conduit, and observing a variation of the pressure in the conduit. For example, an evaporative emission control system disclosed in Japanese Patent Provisional Publication No. 5-79408, is adapted to make a leak diagnosis by reducing the pressure in the above described conduit by using an intake vacuum prevailing in an intake pipe portion downstream of a throttle valve during operation of an engine.